The present invention relates to infrared electric liquid heaters, and, more particularly, to infrared electric water heaters of the type in which an electrically heated filament irradiates a stream of water to increase the water temperature.
Water is typically heated electrically by using immersion-type heaters that are mounted within a water-filled vessel. The generic immersion-type heater includes a resistance wire that is contained within a ceramic matrix which, in turn, is sealed within a metal containment sheath. Electric current is passed through the resistance wire with the heat generated by the wire conducted through the ceramic matrix and the metal sheath into the surrounding water. The heat is transferred primarily by conduction to the water immediately adjacent the containment sheath with the heated water forming convection circuits. These types of heaters are generally well-suited for their intended purpose. Since the immersion heaters rely on conduction as their principal heat transfer mode, they cannot be characterized as fast-acting devices and are not suited for in-line, instant-on water heaters where the water temperature is raised as the water flows from an inlet to and towards an outlet.
Various types of in-line, instant-on water heaters have been proposed. In their simplest form, an elongated immersion-type heater can be introduced into a water-carrying conduit to heat the water as it flows in the conduit. Since the heat transfer mode is primarily by conduction, only that water closest to the hot surface of the heater will undergo a temperature rise. In general, traditional immersion-type heater elements are not optimal in this type of application.
Efforts have been directed toward infrared in-line heaters. In the most straightforward organization, an elongated tubular infrared lamp is mounted along a central axis in a water-carrying conduit. The infrared lamp typically includes a tungsten filament supported within a quartz or silica envelope with the exterior surface of the envelope in direct contact with the water in the conduit. The filament, when energized, emits strongly in the mid-infrared range that is believed to be optimal for heating water by radiant energy. The infrared energy irradiates the water flowing in the confined annular space between the outer surface of the quartz or silica envelope that defines the lamp and the interior surface of the conduit. The interior surface of the conduit is desirably reflective to minimize heat transfer through the conduit and maximize radiant energy flux in the annular volume through which the to-be-heated water flows. In this structural organization, the infrared filament functions to heat the water-contacting surface of its quartz or silica envelope so that water contacting that surface is immediately heated by conduction. Also, the infrared filament also functions to irradiate the flowing water and heat the water as the radiation is absorbed by the flowing water.
While the above-described organization appears to represent sound engineering practice, my past experience has shown that the material of the quartz or silica envelope typically used in the infrared lamp cannot structurally withstand the temperature differentials and thermal stresses involved. For example, it is well known that small amounts of water inadvertently splashed onto the surface of a hot halogen lamp will induce transient stress-gradients that can cause immediate and catastrophic fracture of envelope. As can be appreciated, any breaching of the lamp envelope in an in-line water heater context will allow the water to come into contact with the electrical circuit components. My past attempts to develop an instant-on, in-line heater have been thwarted by problems associated with the inability of the envelope surrounding the filament to withstand the stress associated with the introduction of the to-be-heated water onto the surface of the infrared lamp when the lamp is at temperature.
In an effort to compensate for the afore-described drawback, in-line heaters have interposed another quartz or silica tube around the surface of the infrared lamp. An example of this intermediate-tube construction is shown in FIG. 2 of U.S. Pat. No. 4,534,282 issued to Marinoza on Aug. 13, 1985. While this intermediate tube separates the to-be-heated water from the envelope of the infrared-emitting lamp, the intermediate tube also diminishes the thermal efficiency of the device. My experiments using an intermediate tube surrounding the lamp have demonstrated that the intermediate tube structural organization was not efficient enough for use as an in-line, instant-on water heater. Accordingly, the nature of the quartz or silica material that has historically defined the envelope that surrounds the infrared-emitting filament has prevented the fabrication of a practical, high-efficiency, instant-on in-line heater in which the water to-be-heated is in direct contact with the surface of the envelope that surrounds the filament.